1. Field of the Invention
The present invention relates generally to a rotational speed control method in a process for stopping a gas turbine, and specifically to an operational method for controlling a rotational speed in the stopping process so as to avoid excessive stress to be caused in a moving blade.
2. Description of the Prior Art
FIG. 3 is a perspective view of a gas turbine moving blade, wherein numeral 11 designates a moving blade, numeral 12 designates a platform thereof and the moving blade 11 is rotated by a high temperature combustion gas G in a direction R. In such a gas turbine moving blade, operated at a rated rotational speed, when a load decreases to no load and fuel is shut off for a stop of the operation, then excessive thermal stress and centrifugal force arise in the stopping process, as described later, and a crack may occur in the blade.
FIGS. 2 are explanatory views of the transition of the occurrence of stress in the blade in the above mentioned gas turbine stopping process. FIG. 2(a) shows a load state, FIG. 2(b) shows a rotational speed state, FIG. 2(c) shows a metal temperature state and FIG. 2(d) shows the state of stress at the point A of the moving blade 11 of FIG. 3. In FIG. 2(a), a gas turbine is operated with 4/4 load (full load) until time t.sub.1, on the time axis. Fuel is throttled starting from the time t.sub.1 to time t.sub.2, when the load decreases to 0/4 load (no load). A gas turbine rotor is kept rotated in a state of no load until time t.sub.3 when the fuel is shut off, and then the load comes to a zero state rapidly.
In FIG. 2(b), corresponding to the load transition of FIG. 2(a), the gas turbine is usually kept operated at a rated rotational speed from the time t.sub.2, when the load becomes 0/4, to the time t.sub.3, when the fuel is shut off. When the fuel is shut off at the time t.sub.3, the rotational speed then decreases rapidly to come to a stop.
In FIG. 2(c), the metal temperature is shown with respect to point A of the moving blade 11 and point B of the platform 12, both shown in FIG. 3. As the high temperature combustion gas flows at a constant rate until the time t.sub.1 and likewise the load is the 4/4 load and the rotational speed is the rated speed until this time t.sub.1, the metal temperature is kept at a high temperature level. When the load starts to decrease at the time t.sub.1, the fuel is then throttled starting from the time t.sub.1, and the metal temperature goes down until the time t.sub.2 of the 0/4 load to then be kept constant until the time t.sub.3 while the state of the 0/4 load continues. As the thermal capacity is larger in the platform 12 than in the moving blade 11, the metal temperature is kept higher at the point B than at the point A until the time t.sub.3.
When the fuel is shut off to zero at the time t.sub.3, the metal temperature decreases rapidly at both the points A, B. In this process, while the rotational speed also decreases gradually, at time t.sub.4, when the rotational speed has not yet decreased sufficiently, the differential temperature between the point A and the point B reaches a maximum, and thereafter the temperatures at the respective points decrease gradually to come to the same final temperature.
FIG. 2(d) shows a state of stress at the point A of the moving blade 11. The stress is constant until the time t.sub.1 and then decreases slightly as the load decreases to the time t.sub.2 when the load becomes the 0/4 load. Thereafter, even in the state of no load from the time t.sub.2 to the time t.sub.3, the stress decreases slightly further. However, at the time t.sub.4, when the largest differential temperature .DELTA.T occurs as shown in FIG. 2(c), an excessive thermal stress is generated. In addition to this thermal stress, as the rotational speed still exists to some extent, a centrifugal force in proportion to the rotational speed squared acts. Hence a large force is added to the point A, and a crack may occur, as the case may be, to break the blade.
As mentioned above, in the process oft the gas turbine operated with a full load being decreased in loads so as to be operated with no load, the fuel is shut off and the rotational speed decreases. As the rotational speed does not sufficiently decrease, there occurs a large differential temperature between the moving blade and the platform, and thereby a large thermal stress occurs in the moving blade. Further, in addition to this thermal stress, a centrifugal force in proportion to the rotational speed squared acts. Thus, if such process is repeated, the blade may be broken. There had been no countermeasure in the prior art for preventing the large force acting in the moving blade in this process, and an appropriate countermeasure has been long desired for safety purposes as well.